Wednesday, April 04, 2012

How hard can giant crocodiles bite?

A recent paper published in the journal PLos ONE called "Insights into the Ecology and Evolutionary Success of Crocodilians Revealed through Bite-Force and Tooth-Pressure Experimentation" presents the results of a study that was started over a decade ago measuring and comparing the bite force of different crocodilian species. I've been waiting for this to appear for years (I can almost say decades!) as I was superficially involved in the Australian work including catching wild C. johnstoni that were used. I'd also designed a very low-budget bite force meter and measured C. porosus bite force prior to this, and obtained the measure of 3,800 lbs with a 15.1 ft (4.6 m) captive saltie housed at Crocodylus Park which came in pretty close to the more accurate measurement obtained for this paper. So we've known for a while that these kinds of forces were normal for large crocodiles.

But one of the most interesting, and unexpected findings of this work was that bite force isn't dictated by jaw size and shape significantly. It plays a role, but there's no clear pattern. What does influence bite force primarily is body mass (and therefore size). That shouldn't be surprising - the bigger the croc, the harder it bites. But slender-snouted species like C. johnstoni can put the bite on as well as C. porosus can at similar sizes, which is a very cool finding. Gharials let the longirostrine side down a little, with a lower bite force relative to body mass, but they compensate for having a very specialised jaw design and also by being extremely cool.

Of course one of the reasons I've been waiting for this paper is to find out the relationship between body size (or mass in this case) and bite force with a good-sized dataset. With that information, we can start to ask some interesting speculative questions. For example, how much bite force could the world's largest crocodile generate? Let's try this with Lolong shall we? The equation used is as follows:

(Y(force, N) = 29.632x(body mass, kg)+569.35

So Lolong's mass is 1,075 kg, which means his bite force is estimated at 32423.75 Newtons, or 7289.1 lbs (about 3.3 tonnes of force!). Wow.

But what about Deinosuchus riograndensis, at an estimated 11 metres (36 feet) and therefore one of the largest crocs that has ever lived? Using figures for living crocs, his body mass is estimated at 3,450 kg, and therefore his bite force could have been a staggering 102,803 Newtons (23,102 lbs) which is 10.5 tonnes of bite pressure. Holy cow! No wonder these guys were capable of snacking on dinosaurs.

This is definitely fascinating work and contrary to what I always thought. Don't crocodiles with large, bulky jaws tend to have a heavier mass anyway?

I especially like that just by obtaining a crocodile's bite force you could calculate its weight. According to the formula that 4.6 saltwater crocodile that had a bite force of 3800 would have a mass of about 551 kilograms or 1215 pounds. Do you know the weight of that crocodile? And if you do, is it near my calculation?

Sorry, I forgot to add in my last comment that I was unsure what exactly is measured when obtaining a crocodile's bite force. For example, many times when I have seen people calculate the bite force of a crocodilian they usually measure the back jaws. Is this formula an accumulation of the crocodile's front and back jaws?

Hi Baatman74, you'd better send me an email if you'd like to discuss it confidentially. You can reach me at abritton /at/ crocodilian /dot/ com. Whether a broadcaster would be interested in funding a documentary will depend on a variety of things, but I obviously can't comment any further until you tell me a bit more about it.

Hi ElectricFuneral, well to a degree, but some longirostrine species can grow to very large sizes - both gharials and Tomistoma are reputed to reach almost 20 feet. The study indicates that jaw muscle mass and hence bite potential increases with size, and then the actual force delivered to the prey is further influenced by jaw shape and tooth design. But it's not the other way around, it would seem - jaw design and tooth design don't appear to influence potential bite force to the degree previously thought. There are some criticisms I have about a few aspects of the methods and assumptions made, but they shouldn't have much bearing on this overall conclusion. I'd also like to read the paper again and think about it some more.

Bear in mind (and this is related to one of my concerns about the method) that bite force can be varied tremendously by a crocodile. It may exert anywhere between 1% and 100% of maximum bite force, so you can't reverse the equation - you don't know whether the croc is biting at 100% of bite force to reliably estimate its mass. Length and overall body dimensions are a much better variable to use in that case, although even then the margin of error is high because body condition is important. No, I don't know the weight of the 4.6 m crocodile, it was a captive animal and somewhat fat (like nearly all captives) so even if it was biting maximally you'd likely underestimate its mass.

Bite force as used in the study is less a measure of the actual bite and more a measure of the force being generated by the muscles. Logically, if you apply force to a sharp tooth you'll generate a higher point pressure than applying that same bite force to a blunt tooth. So measuring point pressure is interesting, but doesn't tell you much about the power being generated. The way force is calculated depends on pressure measured, distance to the hinge, and tooth sharpness. You'll find more details about it in the paper.

Romulus Whitaker measured a crocodile slide at almost 4 feet in girth. Was wondering if you watched the program Killer Crocs on the History Channel. Reputedly, there are a couple of crocodiles at the Bhitarkanika National Park in the state of Orissa in Eastern India, said to be over 20 feet. Have you spoken to Mr. Whitaker about his? Why don't you do a study and photographic documentation of the crocodiles there? There are some extremely large adult males and would surely be worth your while.

Hi axl, Rom and I have written a paper (which is currently being reviewed by a journal) about large crocs, and we discuss some of this in there. The main problem with that Bhitarkanika croc (the 23 footer) is that it was a visual estimate and not an actual measurement. As for the slide, yes that was impressive and obviously a big croc, but it could have been anything from 17-18 feet upwards, it's really hard to be sure. Fat crocs give much wider slides than thin crocs, for example, and even the consistency of the mud can play a role.

I would love to do a study on those crocs with the guys there plus Rom. Maybe it will happen if we can get some interest and funding.

Thanks. Some great info there. As it stands, Lolong still rules, yet there is a larger crocodile supposedly where he was captured, witnessed by all those involved. Have you any information from the people there? That part of Asia is proving it's mettle when it comes to giant crocs and am quite sure there are monsters in Indonesia, Sumatra, Borneo and Malaysia. What have you heard so far?

Hi axl, I didn't mean to remove that last comment, but to answer it if you'd like to email me I'm at abritton /at/ crocodilian dot com

As for that other large croc, yes I've seen the photos that were taken of it. There is no sense of scale in there, so it's impossible to estimate size with any accuracy, but I can tell based on the shape of the body, the size of the scutes on the back of the next, and the back of the skull that it's a big croc, but it could be anywhere from 17 to 23 feet long, who knows. I'm not saying that I think it could be 23 feet long, but I'm saying you're really in guesswork territory with it.

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I'm a zoologist specialising in crocodiles, whether it's research, management, conservation or film-making. I run the popular website crocodilian.com and a consultancy business called Big Gecko with my wife Erin.